Phase diagram is a diagram showing the physical phase changes of a substance under various conditions of temperature and pressure. Phase diagram of water is a pressure-temperature diagram showing the changes of water physical phase under various conditions. A simple water phase diagram is shown in the figure below.
The phase diagram, whatever the substance, generally uses the pressure parameters for the Y axis and the temperature parameters for the X axis. The diagram are formed by three curves which become the boundary of the three phases of water physics. Those curves become an equilibrium line of two different water phase that could be exist at the same pressure and temperature.
The first curve becomes the boundary between the solid phase and the gas phase, the second curve being the boundary between the solid phase and the liquid, while the third curve becomes the boundary between the liquid phase and the gas. This third curve also known as saturated line. The phase of water at this line is saturated steam. Along the saturation line, regardless of the magnitude of pressure and temperature, water and steam may be present in equilibrium.
So, if we examine in more detail, the phase of water vapor alone can be classified again into three: saturated steam, superheated steam, and supercritical steam.
The three curves that form the water phase diagram meet at a point known as a triple point. The triple point is a point where the three phases of water physics, ie solid, liquid, and gas, can be in one equilibrium condition. This condition is at a pressure of 0.61 kPa and 0.010°C. Seen in the diagram above, below the triple point, water has no liquid phase. Under this triple point the solid water (ice) will instantly evaporate into gas if there is a rise temperature at constant pressure.
Above the triple point, there are two branching curves with each function. One curve limits between solid and liquid phases, and the other curves constrain between the liquid phase and the gas. At the end of the liquid-gas curve there is a point we know as a critical point. Critical point is a point that becomes the end of the liquid and gas phase equilibrium curve so that it can be in the same condition of pressure and temperature. Critical point of water is at 22.1 MPa and 374°C.
Water vapor at pressures and temperatures over critical points can not be condensed only by increasing the pressure alone. Or another example, if we heat water at a constant pressure of 25 MPa, liquid water will not experiencing saturated steam phase – which is a mixture of water and water vapor – but will instantaneously turn the phase into supercritical vapors. This concept is the basis for the use of supercritical boilers in modern steam power plants.